When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal that is emitted by the excited spins after the excitation signal B1 is terminated.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MRI signals is received using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
These signals can be decomposed into directional components. The relaxation time T1 is the time required for the z component of M to return to 63 percent of its original value following excitation. It is also referred to as spin-lattice relaxation or longitudinal relaxation. The relaxation time T2 is the time required for the transverse component of M to decay to 37 percent of the initial value. It is also know as the spin-spin relaxation time or transverse relaxation time.
The ability to depict anatomy and pathology using MRI is dependent on the contrast, or difference in signal intensity between the target and background tissue. In order to maximize contrast, it is necessary to suppress the signal intensities of the background tissues. Substances can be contrasted in an MR image by the differences in either their T1 or T2 characteristics.
Contrast agents can be used to modify the T1 or T2 characteristics in vivo. The specific modification in contrast caused by a given contrast agent is due to an effect of shortening the relaxation time T1 and/or T2 of the hydrogen nuclei. If the contrast agent reduces T1, a T1 hypersignal is observed in the reconstructed image. On the other hand, if the contrast agent shortens T2, a reduction in the T2 and T2* signal will be observed in the reconstructed image.
One very common application of MR imaging is in vivo screening for tumors. However, distinguishing a tumor from surrounding tissue can, at times, be difficult. That is, it is often difficult to maximize the signal intensity received from the tumor while suppressing the signal intensities received from the surrounding tissue.
Also, generally speaking, MR imaging is advantageous in performing anatomical analysis. That is, unlike other imaging modalities, such as positron emission tomography, MR imaging is not as readily suited for functional imaging. To perform functional analysis using MR imaging, a contrast agent is typically employed. For example, when performing functional MRI (fMRI) of the brain, oxygen is typically employed as a contrast agent using the BOLD method. On the other hand, when attempting to image arterial or venous flow, a contrast agent such as gadolinium is utilized. In either case, the functional imaging is achieved by monitoring the presence or absence of the contrast agent using MR imaging. However, there are many functional processes within the body that cannot be imaged using traditional contrast agents.
Therefore, it would be desirable to have a system and method for enhancing the contrast of specific structures, such as tumors. Furthermore, it would be desirable to have a system and method for imaging functional processes in vivo that cannot otherwise be imaged using traditional functional MR imaging.